Hot air furnace



P 1935. F. H. CORNELIUS 2,013,136

HOT AIR FURNACE Filed June 3, 1951 6 Sheets-Sheet 1 33 INVENTOR l Fran/K HCome/fl/s.

WITNESS ATT Sept- 1935- F. H. CORNELIUS 2,013,136

HOT AIR FURNACE Filed June 3, 1931 I 6 Sheets-Sheet 2 I .62 ll 35* $3- a2 will WITNESS v INVENTOR fia/ H Came/lbs.

ATTOR Sept. 3, 1935. F; H. CQRNELIUS 2,013,136

1101' AIR FURNACE Filed June 5; 1931 6 Sheet s-Sheet 3 INVENTOR Bank H Corne/fus.

WIT NESS'I HOT AIR FURNACE Filed June 3, 1931 e Sheets-Sheet 4 WITNESS: 68 INVENTOR d Fran/6 H Corrie/ms.

4x. my. BY

M My ATTORNEY :7

Sept. 3, 193 F. H. CORNELIUS 9 9 5 HOT AIR FURNACE Filed June 3, 1931 6 Sheets-Sheet 6 INVENTOR Fun c H Carve/ms.

WITNESS 1 w Patented Sept. 3, 1935 I UNITED STATES PATENT OFFICE 6 Claims.

My invention relates to furnaces, and it has special reference to domestic furnaces of the warm air type.

One object of my invention is toprovide a furnace of the character indicated. wherein a heat exchanger is located above a combustion chamber and has one portion directly communicating therewith, together with a blower located to one side of the combustion chamber for directing ventilating air against another portion of the heat exchanger, the entire apparatus being contained within a single casing to form a unitary furnace.

Another object of my invention is to provide a device of the character just set forth, together with an exhaust pipe located above the blower within the furnace and a return ventilating air intake located above the pipe whereby the returning air stream receives heat from the pipe before being drawn into the blower.

A third object of my invention is to provide two spaced doors for effecting access to a combustion chamber, together with a container for fluid to be evaporated located above the space between the doors.

A further object of my invention is to provide apparatus of the character just set forth, together with automatic float means for maintaining a predetermined level in the evaporating chamber.

A further object of my invention is to provide a furnace having an air filter or screen interposed between the return air intake and a blower, together with apparatus for indicating the difference in pressure on the respective sides of the screen, thereby indicating the degree of cleanliness thereof and when the foreign matter should be removed from the screen.

Another object of my invention is to provide a novel closure for the edges and corners of a rectangular object, such as a furnace casing, whereby no screws or the like are required at any point except the corners and a relatively compact double closure is provided at the corners with a minimum of screws or the like.

Another object of my invention is to provide a timing device in combination with a thermostat or other control for afurnace for intermittently operating the furnace blower independently of temperature conditions.

A further object of my invention is to provide individual heating systems for the apartments of an apartment house, thereby enabling each individual tenant to regulate the temperature of the rooms of his own apartment, in accordance with his own preferences and independently of the temperatures of other parts of the building.

Other objects of my invention will become evident from the following detailed description taken in conjunction with the accompanying A drawings, wherein Figure 1 is a view in longitudinal section of a furnace constructed in accordance with my present invention,

Fig. 2 is a view in front end elevation, parts broken away, i 1,

Fig. 3 is a view in rear elevation, with parts broken away, of my furnace,

Figs. 4, 5 and 6 are views partly in elevation 15 and partly in section of my new closure or fastening device for the furnace jacket,

Fig. 7 is a diagrammatic view of the preferred system of control to be utilized in accordance with my furnace, 2

Figs. 8 and 9 are views partly in elevation and partly in section of a regulating apparatus or switch used in my furnace.

Figs. 10 and 11 are semi-diagrammatic views of apartment rooms fitted up in accordance with the present invention, and

Figs. '12, 13, 14 and 15 show modifications of the apparatus illustrated in Fig. 1 for indicat ing the degree of cleanliness of the air filter.

Referring to the drawings, the structure here shown comprises a furnace having a lower compartment l for housing a suitable combustion chamber 2, a compartment 3 located directly above the compartment l for housing a multitubular heat exchanger 4 for transmitting warm air through an outlet 5 to the rooms being heated, a blower compartment or chamber 6 located to one side of the combustion chamber compartment l and housing a suitable centrifugal fan or blower l, and a compartment or chamber 8 above 40 the blower through which a smoke breeching or exhaust pipe 9 extends, an air filter I 0 being located between the blower I and the smoke breeching 9, and a return air intake or bonnet ll being located at the top of the compartment 8.

A wall or partition l5 extends transversely of the furnace, separating the compartments l and 3 on the one hand and 6 and 8 on the other, this partition being provided with a suitable opening l6 near its lower end for permitting air to be emitted from the blower I to be circulated and an opening I! near its upper end through which the smoke breeching 9 extends.

The furnace walls in the vicinity of thecombustion chamber 2 and heat exchanger 4 are 66 with 10 of the furnace shown in preferably of the heat-insulated type, the lower or bottom wall l8 being shown as double in character and a stream of air being directed between the two walls from the blower 1. The other walls, such as l9 and 20, are likewise double in character, and are preferably filled with some suitable heat-insulating medium M. The remaining walls are shown as comprising single plates. The outer corners where three walls of the furnace meet are closed by my novel appa-- ratus 2i, as subsequently described.

The combustion chamber 2 is shown as provided with a suitable burner 22 for gas, and the like, which is supported at an angle upon a perforated plate 23, through which air may be drawn for combustion purposes. A gas intake pipe 24 leads to an electrically-controlled gas valve 25, of a familiar type, on the other side of which the fuel supply pipe 26 extends to any suitable source, as will be understood. Any suitable pilot light device (not shown) may be utilized as is customary.

Direct access to the combustion chamber 2 through a single door is purposely prevented in accordance with my present invention and, in-

stead, a double door structure is utilized. The

outer door 21 is hinged at its upper end and is provided with louvres for admitting air. Opposite the outer door 21 is an inner door 28 in the wall of the combustion chamber, this door being likewise hinged at its upper end, and a chain 29 extends between the lower portions of the two doors. In this way opening of the outer door 21 necessarily precedes and causes the opening of the inner door 28, and the outer door remains relatively cool. A substantial air space is provided between them and the current of indrawn air, as indicated by the corresponding arrows, passes through the louvred door 21 into the space between the doors 21 and 28 and thence downwardly to the triangular space under the gas burner 22. The amount of air delivered to the combustion chamber may be adjusted by means of the movable damper plate l2 and its fastening bolt and nut assembly i3.

A water pan or humidifier 30, which is shown in Fig. 2 as comprising three separate sections, is located above the space between the two doors 21 and 28 and has its inner wall in heat-exchanging relation with the corresponding wall of the combustion chamber 2. The upper end of the humidifier 30 is open to permit the steam from the humidifier to pass in contact with and between the tubes of the heat-exchanger 4, thus permitting the proper percentage of moisture to be entrained in the air driven by the blower 1 through the outlet 5.

To permit the flow of liquid to all sections of the humidifier 30, the illustrated partitions are perforated near the bottom. In order to maintain the desired level of liquid, a float valve apparatus 32 is provided in connection with the water supply pipe 36. This apparatus comprises essentially a float ball 33 of a familiar type and a valve 34 associated with a suitably supported lever 35. The humidifier 30 is shown as integral in character, having a lower wall 38 serving to close access from above to the space between the two doors 21 and 28 and thus forming part of the door frame.

It will be seen that my design of humidifier or water pan, in which the water is boiled or transferred into steam and mingled with the entrained air, adds its moisture to the air stream in direct proportion to the heat or B. T. U. delivery of the furnace. Since such heat delivery is in turn directly proportional to the temperature difference between the inside and outside of the building to be heated, it is apparent that the moisture delivered thus varies directly with the outside temperature.

In evaporating pans old in the art, the amount of moisture absorbed by the air varies directly in accordance with the area of water surface exposed, and the heat of evaporation is taken from the air itself and is a function of the vapor pressure and of the air temperature. My humidifying pan, on the other hand, has a large area heating surface exposed to the combustion chamber, but is relatively narrow from front to rear, resulting in a small water surface being exposed to air and a relatively small volume of water in the pan at any time.

By adjusting the float ball apparatus 32 to vary the'level in the water pan, the mount of moisture to be delivered to the air may'be regulated and this regulation will depend in part upon the relative proportions of recirculated and of fresh air, the latter requiring more moisture than the recirculated air.

The upper portion of the combustion chamber 2 is relatively constricted in area and. communicates through a suitable coupling 39 with a corresponding lower section of the heat exchanger 4, which is composed of a large number of flattened Welded tubes 40, the stream of gases being divided up to flow through the various tubes and eventually into a manifold 45 located at the rear of the heat exchanger and to the upper end of which the smoke breeching or exhaust pipe 9 is attached.

By the use of these flattened tubes a high degree of heat exchange with the air streams, which pass between the tubes and up to the outlet 5, is effected, and to still further increase the heat exchange, individual baffle members M and 44 are inserted into the flattened tubes 40 at spaced intervals, the baffles 4| extending from the bottoms of the tubes at an intermediate point corresponding to the juncture of the rear wall of the combustion chamber 2 with the heat exchanger 4. The front baflies 4| extend somewhat more than half way up into the tubes, forcing the heated gases to travel, first, upwardly and then to the right. The other or rear bafiles 44 extend from the upper wall of the heat exchanger 4 downwardly through the tubes for about threequarters of the distance, thus forcing the stream of gases to pass below the lower edge of the bafiies before entering the manifold 45. The eifect of these bafiles is, of course, to provide a tortuous path for the heated gases, thus forcing them to deliver nearly all of their heat values to the air flowing between the respective tubes, or, in other words, providing a relatively high rate of heat transfer per unit of surface.

For the purpose of holding the various baffles in place, a plurality of bolts or rods 42 extend transversely of the furnace, the tubes being properly spaced along the rods by means of individual collars or sleeves 43 surrounding the rods between the respective pairs of adjacent tubes.

The smoke breeching 9, from the upper end of the manifold 45 to the joint indicated by the line 48 in Fig. 1, is coextensive with the heat exchanger, that is, it extends from one side to the other of the furnace and comprises a plurality of flattened tubes, as indicated by 46 and 41, whereby the incoming or returning air stream through the intake H flows around and between these tubes, thus extracting still further heat values from the combustion gases before the air is drawn into the blower I. As a result, the gases when they finally reach the flue or exhaust pipe proper 49 have a relatively low temperature, sothat'a minimum of heat is lost by the gases passing up the flue.

Just beyond the joint 48 the flue pipe 49 connects with the central portion of the smoke breeching 9 through a suitable manifold 31, in which is located a switch 50 having a thermostatic element 93, the purpose of which will be hereinafter described. It will be noted, however, that the thermostatic element 93 is in contact with the heating gases and thus measures the temperature thereof.

The blower 1 is of a familiar type having a central intake opening 52 and a delivery pipe or outlet 53, which is flexibly coupled to the partition or intermediate wall l5 to prevent the mechanicalvibration from the fan being transferred to the'furnace structure proper. Furthermore, the blower is mounted on cork bases 55 to still further deaden noise and vibration. The blower may be driven by a suitable electric motor 54, shown in Fig. 3, which is mounted directly on the blower structure and is thus totally enclosed by the furnace walls. This arrangement has the advantage of preventing outside dirt from reaching the motor and, at the same time, any heat engendered in the motor is transferred to the ventilating air stream.

The screen or filter I is shown as mounted on suitable angles or straps 56 inclined at an angle above the blower 1 and attached to the intermediate wall or partition l and the rear wall of the furnace. The screen proper preferably comprises a six-sided cage packed with a suitable filtering medium thus insuring that a restricted passage for all incoming air will be provided.

A manometer, or the like, may be provided for the purpose of readily indicating to the furnace owner or attendant when the screen II] should be cleaned. This result isaccomplished by having one end 58 of the manometer located above the screen i0, while the lower end 59 is located just below it. The bent intermediate portion 51 of the manometer, containing a suitable liquid, is located outside of the furnace, as shown in Fig. 3. By suitably calibrating the glass tube, the difference in level of liquid in the two arms of the bent portion, as caused by the difference in air pressure above and below the screen, may be used to clearly indicate the degree of cleanliness of the screen. In other words, when the difference of pressure between the upper and lower sides of the screen, as indicated by the two liquid levels, has reached the desirable limit, the furnace owner or attendant knows that he should remove the screen and clean out the foreign matter, as otherwise dust and dirt will pass through the filter due to the increased filter resistance or higher pressure differential across the filter.

The amount of dirt in the filters may also be indicated or recorded by the change of temperature of the air at the warm air outlet, due to the variation of volume of air delivery with change of filter resistance while the fuel con sumption remains constant.

Fig. 12 shows a modification of my apparatus for indicating the degree of cleanliness of .the air filter I 0. It comprises a thermometer device l30 having a bulb portion l3| extending through the wall I 9 into the portion 3 of my furnace near the maximum temperature control member 72.

device 62. In this way the bulb l3l is always at a temperature corresponding to that of the warm air being delivered to the outlet 5.

The thermometer device I30 also has a portion I32 vertically positioned just outside the wall l9 and bearing graduation marks indicating degrees of temperature. In order that the maximum temperature may be recorded as long as desired, the thermometer device is preferably of the clinical type, having a constriction I33 in the inner passage housing the mercury, or the like, just above the bend I34.

As the temperature of the bulb I31 gets higher and higher in accordance with the amount of. foreign material clogging the air filter ID, as explained above. this fact is recorded on the portion I32, until finally a mark, such as 170, is reached, which indicates such a clogging of the filter as to require a cleaning thereof. this temperature is reached, therefore, it furnishes a clear indication to the owner or attendant of the furnace that the filter should be cleaned.

From the electrically-controlled gas valve 25, of any suitable construction, a conduit 50 containing the necessary electrical wires extends downwardly and across the bottom of the furnace, and a second conduit 5| then carries the necessary wires upwardly to a maximum temperature control apparatus 62. In order to carry the necessary wires to the blower motor 54, suitable conduits 64 and 65 extend over the top and When down the rear side of the furnace, respectively,

as shown in Fig. 1.

The direction of the heated gases is shown in Fig. 1 by the solid arrows. They emanate from the burner 22, pass over the baffles M in the heat exchanger 4 and under the bailies 44, thence into the manifold 45 and the smoke breeching 9 and finally into the exhaust pipe 49.

The air stream is indicated by the dotted arrows, and is shownas entering the return air intake ll, whence it passes between the tubes constituting the smoke breeching 9, through the filter i0 and thence into inlet 52 of the blower 1. The delivery end 53 of the blower 1 forces a small stream of air under thecombustion chamber I so as to dissipate the heat from the front of the combustion chamber and access door frame, while the greater portion of the air stream is initially blown against the rear or cooler end of the heat exchanger 4, which is preferable, from the standpoint of higher efficiency, to having the coolest air strike the hottest portions of the heat exchanger, as is done in prior furnaces. This air stream traverses the spaces between the flattened tubes 40 and mingles with the moisture derived from the humidifying pan 3!), the combined stream making its exit through the outlet pipe 5 to the various rooms of the house, whence the stream returns to the intake l l and the recirculation begins.

Figs. 4, 5 and 6 show the fastening or closure means for the walls of my furnace. The top wall 20 and the side walls 68 and 69 are positioned at right angles to each other, as shown inFigs. 1, 2 and 3, and any gap or break in the walls, such as shown in the wall 69, may be closed without thenecessity of any screws or other fastening devices by means of a structural member 10. This member 10 has a wide flat outer surface, with which is integrally associated an S-shaped channel member H and a reverse S-shaped channel In each case the lower part of the Ss is flattened so that these parts are in contact with the sides of the wide portion III, while the remaining parts of the S-shaped portions provide grooves or openings I3 and I4, respectively, in which the edges of the adjacent parts of the wall 59 closely fit. In this way a closure for any such gaps of the side walls is produced.

With respect to the corners, where the three walls come together, in the first place, members I5, I6 and 11, corresponding to the member I8, but suitably bent or curved, are used to receive, and close the gap between, the edges of each adjacent pair of the three walls. By continuing these edge closure members until they contact at their corners, a substantially triangular space, as shown in Fig. 5, is left open at the very corner, and it will be noted that the outer corners 18, I9 and 89 of the walls 20, 69 and 68, respectively, protrude beyond the engaging grooves in the respective edge closure members. These projecting corners are properly apertured to receive suitable screws 84, and 86 which project through the corners into apertures 83 in an inner bent plate 8| roughly triangular in outline, which thus serves to connect the inner walls of the furnace at the corner and close the gaps between them.

To fully enclose the outer opening, the outer cap 2I, shaped to conform to the outer surfaces of the edge closing members I5, etc., is attached by a single screw 81 that passes through an opening 82 in the inner plate 8|.

In this way, as clearly shown in Fig. 4, a furnace of very pleasing and smooth outer appearance is provided with only one screw, such as 81, showing from the outside at each corner. In addition, only three more concealed screws are needed at each corner for fastening all parts together securely and efficiently. The general appearance of a stamped one-piece construction is thus provided.

Figs. 8 and 9 show the type of switch that may be used as the switch 58 shown in Fig. 1, for example. A suitable case 90 is provided which has a tubular extension 9| on one side through which a torque rod 92 extends. A helically wound strip of a suitable bi-metallic element 93 has its ends respectively attached to the bent end 94 of the rod 92 and a portion 95 of the tubular extension 9|. The rod 92 is provided with a slip clutch 96 at its inner end to transmit movement through a second rod 98 operating within a bearing 91 to a clamp 98a on a mercury switch tube 99.

The mercury switch is provided with two contact terminals IIII and I02 which are covered by a pool of mercury I03 in the on or circuit-closing position, as illustrated in Fig. 9, thus completing a circuit through the leads I06 and maintaining the controlled circuit closed whenever conditions are suitable.

The mercury switch cannot tilt in either direction through a greater are than that defined by arm I04 and stop members I05, respectively attached to the clamp 98a and a bracket IIII for securing the bearing to the case 98.

Assuming the switch to initially occupy its off position, it will be understood that when the temperature increases sufficiently, the thermostatic device 93 will operate to turn the torque rod 92 in a direction such as to tilt the mercury switch into the position shown, thereby closing the cir-' cuit controlled by the switch by reason of the immers'ion in the pool of mercury of the contact terminals IIII and I02. The mercury tube 99 is permitted to operate through a fixed are due to the lever arm I04 and stop posts I05. This are denotes a definite temperature change of bimetallic element 93. Any temperature change beyond this amount causes a corresponding slippage of the friction clutch 96. The spring IIIII merely holds the clutch discs together. The mercury switch will return to the"o position as the thermostatic element 93 cools by an amount sufficient to effect and permit this.

Referring to the diagram of Fig. 7, this includes the maximum temperature control device 62, the electrically controlled gas valve 25, the blower motor 54 and the switch 50 in the exhaust pipe manifold 31, and, in addition, includes a room thermostatic device III) for regulating the temperature of the heated rooms, and a timing device I I I, which is connected in parallel relation to a hand-operated switch II2 to control the blower motor 54, energy being derived from a pair of supply-circuit conductors H3 and. H4.

The switch 50 is connected across the blower motor 54 and the supply circuit, thus providing three separate means for governing the blower motor, namely, the switch 50, the hand-operated switch H2 and the timing device II I, for a. purpose to be set forth. This switch 50 requires no adjustment or predetermined setting as it automatically adjusts itself. In the warming-up peried or on cycle of the furnace, gravity circulation of the warm air is practically eliminated as the time lag is usually shorter than with other blower or fan switches placed in the warm air bonnet II. This is desirable as it is a known fact that air in a duct system cannot be properly distributed to the rooms by both gravity circulation and fan circulation.

The switch 58, controlled by the temperature of the flue gases, is adapted to operate at a suitable interval, for example, one-half minute, after the gas burner 22 ignites, thus reducing to a minimum this time lag, but assuring a warm furnace before the fan starts and preventing cold air circulation. Further, the blower will be permitted to operate for a suitable interval, say six minutes, after the gas burner is extinguished, during which time the residual heat of the furnace is distributed to the rooms to be heated. These results are not possible in prior systems utilizing a. thermostatic device in the warm air outlet. However, if desired, the switch 50 may be placed in the warm air bonnet and give better operating characteristics than the usual type of blower switch located in the warm air bonnet.

The room thermostatic device II II is connected in series relation with the maximum temperature control device 62 and the electrically controlled gas valve 25. In this way the gas valve is opened to admit fuel gas to the furnace whenever the thermostatic device IIO reaches the point where more heat is desired, while the maximum temperature control device 62 opens the circuit of the electrically controlled gas valve whenever the temperature of the air in the neighborhood of the top of the heat-exchanger 4 reaches a given high figure. In this way the temperature of the rooms is maintained substantially at the temperature for which the thermostatic device H0 is set and the warm air cannot exceed under any conditions motor 54 should fall, themaximum temperature control would shortly break the circuit of the electrically controlled gas valve 25, since the temperature of the furnace would quickly increase if the circulation of air was not affected by means of the blower. Moreover, if the furnace owner or attendant does not pay attention to the illustrated manometer and thus clean the screen I when it should be cleaned, the increased air temperature by reason of the increased air resistance resulting from the clogged filters, will reduce the volume of air delivered by theblower and result in higher temperature within the furnace, which will operate the maximum tempera.- ture control device 62. This will also serve as an indication of the necessity of cleaning the air filters. In this way the'continued intermittent operation of the blower which would occur in connection with old furnaces and thus pull dirt through the filters under the conditions mentioned cannot occur in connection with my furnace.

The chief purpose of the timing device III is to intermittently start and stop the blower motor I4 for ventilating a room or rooms when the room thermostat IIO does notcall for heat. In particular this scheme is adapted for the ventilation of sleeping rooms where the windows remain closed during the night and when the room thermostat is manually or automatically lowered to produce a temperature of, say, 55 F. There are many nights when the furnace would not operate as the house would not cool to this term perature. However, the timing device, by operating the blower motor at predetermined intervals; will'circulate air within the house or bring in fresh air from the outside that will be filtered and automatically kept at a temperature not lower than the 55 figure mentioned above. In this way a room condition is maintained that is desirable from the standpoint of temperature, air motion and cleanliness.

Figs. 10 and 11 show in semi-diagrammatic form the application of my furnace to individual control of the temperature of the rooms in each apartment of an apartment house, In many cases, because of the different preferences. or physical conditions of different tenants, any one temperature is not satisfactory to all tenants, but it is difficult to regulate this if a central heating plant for heating of apartments is provided.

In Fig. 10 I illustrate a plurality of rooms H and H6 corresponding. to different apartments and located on Opposite sides of a central passageway or corridor I I! having a lowered or false ceiling H8, thereby providing a space above the corridor for a purpose to be set forth. The rooms H5 and H6 may preferably be the kitchen,'a1- though this is not necessary. These rooms are provided with doors I20 which open to other rooms of the respective apartments and are provided with grilles I2I, in the lower part of the doors for permitting air to be drawn therethrough into the respective rooms H5 and lit. Each of these rooms is shown as provided with a furnace I22 of the type illustrated in Fig. 1, etc.,

and have an air delivery pipe I23 corresponding to the air delivery pipe 5 of Fig. 1. This pipe is divided, when it reaches the space above the false ceiling II8, into as many branches I24 as there are rooms in the apartment to be heated, and each one of these branches extends to a register, such as I25 or I25, in the respective rooms. As shown in Fig. 11, the register I25 provides warm air for the room H5 in which the furnace I22 is located, while the register I 26 provides warm air for an adjacent room I 21. It will be noted that the other furnace delivery pipe in room I I6 is likewise divided into a plurality of branches that extend above the false ceiling H8, and a central barrier I28 serves to separate the pipes corresponding to one apartment from those corresponding to another.

In this way the individual tenants of an apartment house may regulate their own room temperatures as desired, air being returned to the rooms I I5 and I I6 and so to the furnaces, through the grilles in the bottoms of the doors, and redistributed to the several rooms through the air delivery pipes or ducts. A grille or register I29 may be provided in the wall or partition between the furnace room II 5 and the adjacent room I21, for example, in lieu of the door grilles I2I.

Figs. 13 and 14 show another device responsive to the temperature in the vicinity of the maximum. temperature control device 62, which may be substituted therefor, serving the double purpose of controlling gas valve 25 and indicating the condition of the air filter III. In this case the clutch 95, spring I00, and stop members I04 and I05 may be omitted. The clamp 98a is provided with an extension I36 projecting through a slot I31 in the rear wall of casing 90. The extension I35 is adapted to engage one side of a pointer or indicator I38 pivoted at I39 upon the casing 90. In order to provide a pivotal mounting with sufiicient friction to insure that the pointer I 38 will remain where it is pushed by the extension I36, washers I40 and I are provided on the pivotal mounting I39 of the pointer, together with a spring I42 pressing against the outer washer MI.

The position of the pointer I38 indicated in Fig. 14 is the normal or "clean filter position,

as indicated by that legend, this position correspending to the normal or closed circuit position of the mercury tube 99. As the temperature increases, the extension I36 pushes the pointer towards the other extremity of the slot I31, indicated by the legend Dirty filter. When this extreme position is reached bythe pointer, the owner or attendant of the furnace is givenclear be omitted from the structure shown in Fig. 13.

In their places a disc I43 having a peripheral notch I44 is secured to the torque rod 92. A pawl I45 is pivoted at I46 to the casing 90, and rides along the periphery of the disc I43 as the latter is rotated in response to the variations of temperature aifecting the bi-metallic element 93. When a critical temperature is reached, which corresponds to the position of the notch I44, as indicated by the legend Dirty filter, the pawl I45 falls into the notch, thus preventing further operation of the device until the owner or attendant of the furnace resets the device. He is thus given clear notice of the necessity of cleaning the air filter I0. I

It will be seen that I have thus provided a hot air furnace which is adapted for either central heating plants or individual apartment heating plants, and which is novel in its arrangement of parts and efficient in its extraction of heat from the burning gases, which has automatic control fastened together with relatirely few screws, or the like.

I do not wish to be restricted to the specific circuit connections, structure or arrangement of parts set forth herein, as various modifications thereof may be made without departing from the spirit and scope of my invention. I desire, therefore, that only such limitations shall be imposed as are indicated in the appended claims.

I claim as my invention:

1. An air conditioning unit embodying a heating means and having a substantially constant rate of heat delivery, an air filter therein varying the rate of air fiow in accordance with changes in the air filter resistance, and means responsive to the temperature of a predetermined portion of the air stream heated by said heating means for indicating the degree of cleanliness of said filter.

2. An air conditioning unit embodying a heating means and having a substantially constant rate of heat delivery, an air filter therein varying the rate of air flow in accordance with changes in the air filter resistance, and means responsive to the temperature of the air outlet of the unit, said air being heated by said heating means, for indicating the degree of clogging of the filter.

3. In combination, means of substantially fixed heat capacity for altering the temperature of an air stream, air-filtering means, air-handling means for causing said air stream to pass through said temperature-altering means and air-filtering means whereby the volume of flow and maximum temperature of said air stream vary with the state of cleanliness of said filtering means,

and thermostatic means responsive to said maxi mum temperature for thereby indicatingthe cur rent state of cleanliness of said filtering means.

4. In combination, means of substantially fixed heat capacity for altering the temperature of an air stream, air-filtering means, air-handling means for causing said air stream to pass through said temperature-altering means and air-filtering means whereby the volume of flow and maximum temperature of said air stream vary with the state of cleanliness of said filtering means, and thermostatic means responsive to said maximum temperature for governing the operation of said temperature-altering means upon said filtering means accumulating a. predetermined dirt content.

5. In a warm air furnace, in combination, airtemperature-altering means, means for circulating an air stream through said temperature-a1- tering means, means thermally responsive to flue gas temperature for controlling said circulating means, and timing means for intermittently circulating said air stream when said temperaturealtering means is inactive.

6. In an air-conditioning unit, the combination with temperature-altering means, of means for circulating an air stream through said temperature-altering means, means thermally responsive to the temperature of a discharge fluid from said unit for controlling said circulating means, and timing means for intermittently circulating said air stream when said temperaturealtering means is inactive.

FRANK H. CORNELIUS. 

